The present application is directed to power amplifiers and more particularly to a circuit that protects power amplifiers from operating at excessive voltages that might cause damage to the power amplifier.
Wireless transmitters typically comprise signal processing circuitry followed by a power amplifier that elevates the power of a signal to be transmitted prior to emission by an antenna. Such power amplifiers are becoming more common as a result of the ever increasing proliferation of mobile terminals that incorporate some form of wireless transmitter.
The term xe2x80x9cmobile terminalxe2x80x9d encompasses pagers, cellular phones, personal digital assistants, laptops equipped with wireless modems, and the like. These mobile terminals are subject to numerous telecommunications standards and regulations which govern their behavior. Many of these standards govern the power levels with which the mobile terminals are allowed to transmit signals. This is done to prevent cross-channel interference in some cases, as well as help the mobile terminals conserve battery power. In many instances the power levels are controlled, either by a base station or by the mobile terminal at the instruction of the base station.
One side effect of this mobile environment is that the power amplifier of the transmitter has to withstand large voltage standing wave ratios (VSWR) at elevated supply voltages. GSM in particular may generate a VSWR of 15:1 at times. This can quickly lead to device burnout as voltage peaks during successive periods at collectors of the transistors forming the power amplifiers exceed tolerable voltage levels.
In addition to mobile terminals, wireless LANs of computing devices are also becoming more common. The computing devices include a wireless transmitter that likewise has a power amplifier that may be subject to extended voltage peaks and damage therefrom.
Thus, there remains a need for a way to protect power amplifiers in transmitters from voltage peaks during successive periods that exceed design parameters and may cause burnout in the power amplifier.
The present invention adds a feedback loop to a power amplifier. The power amplifier, in an exemplary embodiment, is part of a transmitter chain. The feedback loop comprises a sensing circuit and a processing circuit. When the sensing circuit senses that the output of the power amplifier is high enough that operation at that operating point for successive cycles or periods would damage the power amplifier, the sensing function causes the processing circuit to change the bias provided to the power amplifier in such a manner that the output power is lessened to an acceptable level.
In a first embodiment, the sensing circuit is formed from a transistor that acts as an avalanche diode. In a second embodiment, the sensing circuit is formed from a plurality of diodes. In a third embodiment, the sensing circuit is formed from a plurality of transistors. Common among these sensing circuits is that they remain off until the output of the power amplifier exceeds a predetermined threshold. Above the predetermined threshold, the current passing through the sensing circuit increases rapidly with small increases in applied voltage above the threshold.
The processing circuit may act on the bias circuit differently depending on different embodiments. In an exemplary embodiment, the processing circuit comprises a current mirror that sinks bias current from the input of the power amplifier, thereby reducing the output of the power amplifier. In a second exemplary embodiment, the current mirror turns off the bias current when the sensing circuit detects an output over the threshold. Alternative actions may take place depending on the nature of the bias circuit used.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.